Molded records, such as conventional audio records or the more recently developed video discs, are manufactured in a process in which a molding composition is pressed between a pair of metal plates referred to as stampers. The stampers have defined in their molding surfaces a spiral groove which contains a surface relief image defined in either the side walls or the base of the groove which corresponds to the information desired to be reproduced on playback of the record. The spiral groove in the record is used to guide the stylus of the playback apparatus. In order to obtain proper playback of the record, it is important that the stylus precisely follow the spiral groove in a continuous manner. If the stylus is caught in a given groove, a condition known as "lock groove" occurs where the record does not play continuously but rather repeats a given portion of the recording. A further type of defect which is encountered is known as "skip groove", where, as the name implies, the stylus skips over a series of grooves causing a discontinuity in playback.
It has been found that many times the defects encounted in playback, such as lock groove and skip groove, are caused by microdefects molded into the record. The microdefects typically are present in the molded surface of the record as either pits or as bumps blocking one or more grooves. The microdefects which are molded into the record are a very serious problem in that records having these defects must be scrapped.
Microdefects molded into a record can often be traced directly back to the stamper used to press the record, in that many times the stamper will be found to have similar mating microdefects defined in the molding surface of the stamper.
Many of the microdefects found in the stampers can, in turn, be traced back to the mold on which the stamper was electroformed. However, surprisingly, a substantial number of the microdefects found in the stampers cannot be traced back to the mold.
In a like manner, many of the microdefects found in the molds, such as pits and bumps, in many cases cannot be traced back to the master on which the molds were electroformed.
Based on the progressive appearance of the microdefects from the master to the molds and from the molds to the stampers, it was thought that the microdefects were caused by some problem which occurred during the step of electroforming the molds on the masters and the stampers on the molds. Those skilled in the art, however, were unable to specifically identify much less correct the problems in the electroforming procedure, and molded records exhibiting lock groove, skip groove, and the like caused by the microdefects, such as the pits and bumps continued to be a major cause of rejection of the molded records.
Accordingly, it would be highly advantageous if an improvement in electroforming processes could be found which would eliminate or substantially reduce microdefects.